1. Field of Invention
The present invention relates to surface light source devices of the side light type employing in combination a light guide plate with emission directivity and a prism sheet, and more particularly to surface light source devices of said type improved to provide bright output illumination light having enhanced degree of polarization in the front direction. A surface light source device of the present invention is adaptable for various applications requiring uniform, polarized illumination light as well as frontal emission directivity.
2. Related Art
Surface light source devices of the side light type are provided with a light guide plate and a light source element such as cold cathode tube lamp. The light source element is disposed along one minor surface (side surface) of the light guide plate. One frontal major surface defines a light emission plane for outputting illumination light therefrom. The surface light source devices of this type have been widely employed to provide back-lighting for use in liquid crystal display due to output rays of illumination light having relatively large cross-sectional area without increasing the depth size thereof.
As one advantageous light-guide plate material, a light-scatterable optical guide is known in the art. This is an optical element which comprises a transparent optical material and a micro-component with a different refractive index being distributed therein to attain light-scattering capability. Side-light type surface light source devices using a light guide plate made of such light scatterable material are simple in structure and yet good in efficiency for light utilization. In addition, it is possible for rays of output light from the emission surface (front surface) to exhibit enhanced directivity under certain condition that the transparent optical material is well controlled in size of internal non-uniform-refractivity structure to prevent it from being excessively small (for example, 0.06 .mu.m or less). Such light guide plate satisfying the above condition will be referred to as the one with "emission directivity" hereinafter.
Another type of light guide plate with emission directivity for use in surface light source devices of side-light type is known which has with a fine unevenness on the surface of a transparent light guide plate for suppressing occurrence of total reflection thereat. Such added surface unevenness may be attained by forming physical configuration on the surface of light guide plate per se, or alternatively by use of multiple transparent micro-particles adhered by a transparent binder on the flat surface of the light guide plate.
The use of the light guide plate with emission directivity in a side-light type surface light source devices advantageously serves to enhance the brightness when looking at the emission surface from a direction matched to the emission directivity. However, the main propagation direction (referred to as "priority propagation direction" hereinafter) of light rays as output from the emission surface of the light guide plate with emission directivity remains much deviated in angle from the front direction of the emission surface. This angle of inclination may typically range from 60.degree. to 80.degree.. Practical examples will be set forth later.
For instance, since observation of a liquid crystal display is ordinarily done from around the front direction, it will be desirable to provide back-lighting using illumination light that is corrected in priority propagation direction to match the front direction of the display.
One prior known approach proposed recently to achieve such correction of the priority propagation direction for coincidence with the front direction is that a certain element called the "prism sheet" is placed over the emission surface of the light guide plate used. Two modes of prism sheet layout are available: one mode is to dispose the prism sheet with its prism surface facing inwardly (opposing the emission surface), and the other is to dispose it with the prism sheet facing outwardly (opposite to the emission surface).
See FIG. 1 which is a perspective view of the basic arrangement of one prior art surface light source device employing the former mode. Referring to FIG. 1, numeral 1 designates a light guide plate having a wedge-shaped cross-section, which is comprised of a chosen light-scattering optical conductive material consisting of, for example, a matrix made of polymethylmethacrylate (PMMA) and a hetero-refractivity material with different refractive index values as uniformly distributed therein. The light guide plate 1 has a thickness-increased end edge portion for defining a light incidence surface 2 while an associated light source element (cold cathode tube lamp) L is disposed near the edge. A reflector 3 is disposed along one major surface (back surface) 6 of the light guide plate 1, which reflector is made of either a silver foil sheet with specular reflectivity or a white sheet with diffusible reflectivity.
Light as supplied from the light source L to the light guide plate 1 is output from the other, opposite major surface (emission surface) 5. A prism sheet 4 is disposed along the emission surface 5. The prism sheet 4 has a prism array having a number of parallel elongate prism units with fine pitches; it also has a flat surface 4e without such prism array. One prism element of the array is defined by a pair of slant surfaces 4a, 4b.
The prism sheet 4 exhibits transparency, and may be a chosen plastic material such as polycarbonate, for example. Note that in FIG. 1 and the other figures, both the distance between the prism sheet 4 and emission surface 5 and the pitch of any adjacent prisms of the array are exaggerated in scale for illustration. Where the surface light source device is applied to back-lighting of a liquid crystal display, a known liquid crystal display panel will be further disposed outside the prism sheet 4.
In the surface light source device shown in FIG. 1, the light guide plate 1 is constantly decreased in thickness with an increase in distance from the light incidence surface 2. Thus, the characteristics of illuminance efficiency and illuminance uniformity are superior due to the effect of repeated reflections within the light guide plate 1. Such effect based on the wedge shape of light-scatterable optical guide has been fully described in Published Unexamined Japanese Patent Application No. 7-198956.
Light fed from the light source element L into the inside of light guide plate 1 is optically guided to travel toward the opposite side edge 7 of a decreased thickness while undergoing some scattering and reflecting actions. Through this process, light will be gradually output from the emission surface 5. As has been described previously, the output light from the emission surface 5 has a priority propagation direction 5a under certain condition that particles of different refractive index distributed within light guide plate 1 is not extremely small in size (generally, the correlation distance regarding the ununiform-refractivity structure as will be described in detail later). This priority propagation direction 5a is usually inclined approximately by 60.degree. to 80.degree. with respect to the normal to the emission surface 5.
The output light from the emission surface 5 having the priority propagation direction 5a enters from the internal surfaces 4a, 4b of the prism sheet 4 and then leaves the external surface 4e substantially in the front direction. This correction action in priority propagation direction is mainly based on the internal reflection of the prism sheet, which will be explained with reference to FIG. 2 below.
FIG. 2 is a diagram for explanation of light's behavior within the profile along the "perpendicular direction to lamp L." Here, the "perpendicular direction to lamp L" may refer to the "direction normal to the elongate direction of lamp L"; more specifically, it means the "direction crossing at right angles to the elongate direction of the light incidence surface 2." This will be simply called the "lamp-normal direction" hereinafter. Similarly, the "parallel direction with the elongate direction of lamp L"--i.e., the "parallel direction with the elongate direction of incidence surface 2"--will be simply referred to as the "lamp-parallel direction" hereinafter.
As shown in FIG. 2, the prism sheet 4 is placed over the emission surface 5 with its prism-array surface opposing surface 5 directly. The profile of the prism array formed on the prism sheet exhibits a series of isosceles triangles with its apical angle .phi.3 as shown.
Under the condition that light is supplied in the direction as denoted by arrow L', the priority propagation direction of output light derived from the emission surface 5 is inclined with respect to the normal to the emission surface 5 by certain angle .phi.2=65.degree. to 75.degree., or therearound. Supposing that the refractive index of light guide plate 1 is approximately 1.5 (PMMA, 1.492), the angle of incidence .phi.1 to emission surface 5 from the inside thereof is slightly less than 40.degree.. Such specific light rays corresponding to the priority propagation direction will be called the "representative beam" B1.
After passing straight through an air layer AR (refractive index is n0=1.0), the representative beam B1 output from the emission surface 5 is incident onto one slope 4a of the prism sheet 4 and then receives slight degree of refraction action therefrom. Attention should be paid to the fact that the rate of beam B1's incidence to the opposite slope 4b remains relatively small.
The representative beam B1 tends to pass straight through the inside of prism sheet 4 up to the slope 4b; after specular reflection (total reflection) at here, the beam hits the flat surface 4e of prism sheet 4 from its internal side. Assuming that the prism angle .phi.3 is suitably designed by taking account of the output angle .phi.2 from emission surface 5 and the refractive index n2 of prism sheet 4, the angle of incidence .phi.4 to flat surface 4e is about 90.degree., providing illumination light ray 4f which is output from the flat surface 4e of prism sheet 4 to propagate in substantially the front direction.
In this way, with use of the layout arrangement shown in FIG. 1, directional correction for forcing illumination light to collectively travel in the front direction may be accomplished. Unfortunately, however, this arrangement does not come without accompanying difficulty as follows.
As can be seen from viewing FIG. 2 also, this prior art arrangement scheme is principally designed to attain its intended conversion of the priority propagation direction mainly based on the reflecting action at the internal surface of slope 4b, providing output light 4f in the front direction. As a consequence, it remains unexpectable to attain light-collection effect which allows the output light from emission surface 5 to be derived from the surface 4e while enabling collection thereof toward the front direction. This may be reworded in such a manner that in the mode of layout letting the prism surface face inwardly, any light collecting function (condensing lens action) can hardly be expected though the polarizing function may be attained successfully.
As will be presented by use of examples later, light that is actually output from the emission surface 5 of light guide plate 1 has its propagation direction which may exhibit a spreading tendency to certain degree. Accordingly, it can be said that the prescribed layout arrangement is incapable of expecting a satisfiable light collecting function has much room for improvements with regard to outputting of light in the front direction.
In order to let the prism sheet exhibit enhanced light collecting action, another mode has been proposed to turn over the prism sheet 4 as employed in the structure of FIG. 1 in such a way that the prism surface thereof is disposed to face externally.
See FIG. 3 which is a diagram for explanation of the representative beam's behavior where such a mode of arrangement is employed. The prism sheet 4 here is disposed with its slopes (prism surfaces) 4c, 4d constituting the prism array being exposed to the outside. The profile or cross-section of each prism in the array defines an isosceles triangle. Therefore, representing the angles of inclination of these prism surfaces 4c, 4d as .phi.6, .phi.7, the following is given: .phi.6=.phi.7. .phi.5 represents the prism vertical angle.
Under the condition that light is fed as indicated by arrow L', in the same manner as in that of FIG. 2, one representative beam B2 corresponding to the priority propagation direction hits the emission surface 5 at an angle that is slightly less than .phi.1=40.degree.; its greatest portion goes into an air layer AR (refractive index n0=1.0). The output angle .phi.2 in this case approximately ranges from 65.degree. to 75.degree. as has been discussed previously.
The representative beam B2 derived from the emission surface 5 goes straight within the air layer AR; thereafter, it attempts to enter at an angle the flat surface 4e of the prism sheet 4. The representative beam B2 further travels along a specific refraction path shown in FIG. 3 to outgo from the surface 4d of prism sheet 4 (part of it is from the surface 4c in some cases). By adequately designing the prism vertical angle .phi.5 by taking into consideration the output angle .phi.2 from the emission surface 5 as well as the refractive index n2 of the prism sheet 4, it is possible for the propagation direction of output light 4f to be substantially identical to the front direction.
In this way, the arrangement with the prism surface facing outwardly also can correct the priority propagation direction into the front direction. In this layout, each prism of the prism array having the paired slopes (prism surfaces) 4c, 4d may act as a convex lens or its equivalents. As a result, the front-direction collecting action may be effected even for rays of light propagating in directions angularly deviated from the representative beam B2. This means that both the polarizing function and the light collecting function can be achieved simultaneously facilitating outputting of light toward the front direction.
As mentioned above, arranging the prism sheet satisfying specific vertical angle conditions over the emission surface of the light guide plate with emission directivity with its prism array surface facing outwardly, the illumination light flux of the surface light source device is collected around the front direction thereof. However, the inventor's detailed analysis on optimal modes and arrangements of the prism sheet as employed in combination with the light guide plate with emission directivity has revealed the fact that the aforesaid scheme still suffers from several problems that remain unsolved, as follows.
(1) The optimization conditions for achievement of illumination light well collected around the front direction is not given in view of the polarization characteristics of the illumination light. The polarization characteristics of frontal illumination light is an important factor which can reflect directly on the brightness of display images when applied to back-lighting of a liquid crystal display.
(2) Concerning the profile or cross-sectional shape of the prism array of the prism sheet, the prior art approaches are made under the assumption that the profile is a series of isosceles triangles (.phi.6=.phi.7), without taking care of easy processing and manufacture while avoiding degradation in characteristics.
First, an explanation will be given on the viewpoint (1) for polarization characteristics. It has been described that the vertical angle .phi.5 that provides the illumination light 4f travelling in the front direction (more precisely, the angle of inclination .phi.6 of the surface 4d) is directly influenced by the output angle of refraction .phi.2. This in turn necessitates analyzing the intensity of output light from the emission surface 5 with respect to angles for each polarization component in order to consider the polarization characteristics of illumination light. And, the prism sheet is optimized to meet the requirements as derived from the analysis.
FIGS. 5 and 6 are graphs relating to surface light source devices (two examples) using a light guide plate with emission directivity, each of which presents the measurement result of the polarization characteristics of output light from the emission surface of light guide plate with light angle being varied. Measurement conditions used in both graphs are generally similar to that shown in FIG. 4. More specifically, the light guide plate 1 employed in the surface light source devices under measurement is constructed from light-scatterable optical guide having wedge-shaped profile. As the material of this optical guide, a matrix made of polymethylmethacrylate (PMMA with the refractive index of 1.492) was employed with silicon-based resin material (TOSPAL 120; trade name of Toshiba Silicon Co., Ltd.) being uniformly distributed therein as a material of different refractive index.
The composition ratio of silicon-based resin material was set at 0.01 weight percent (wt %) for the light guide plate used in the measurement of FIG. 5 whereas the same for that used in measurement of FIG. 6 was 0.071 wt %. Several conditions including the size of light guide plate 1 are as follows.
depth looked at from the incidence surface 2: 180 mm PA1 width: 135 mm PA1 thickness at incidence surface 2: 2.5 mm PA1 thickness at the end portion 7: 0.5 mm PA1 diameter 1 of cold cathode lamp L: 2.4 mm PA1 distance between incidence surface 2 and cold cathode lamp L: 1.0 mm
The straight-tube like cold cathode lamp L is surrounded on its back side by a reflection sheet R, made of silver foil, for elimination of light dissipation. Silver foil was also disposed as a reflector 3 on a back surface 6 of light guide plate 1. Thin air layer (thickness .delta.1) is present between the silver foil 3 and back surface 6.
Reference character M indicates a luminance meter (Model LS110 by Minolta Co., Ltd. attached with a close-up lens of measurement view angle of 1/3.degree.) used for measurements of luminance. Under the condition that the luminance meter M is set so as to constantly view the center point P of the emission surface 5 along a view line b at a position distant therefrom by 203 mm, measurements were done while causing the direction of view line b to rotate for scanning in a vertical plane with respect to the cold cathode lamp L. The vertical axis in both graphs denotes the luminance value of cosine (COS)-corrected P- and S-polarization component. The COS-correction compensates variations in area of the emission surface under measurement in accordance with scan angles .phi., with 1/cos .phi. being as a factor.
The luminance meter M was with a polarization filter rotatably attached thereto. The polarization filter was adjusted during measurements of P-polarization component to allow such P-polarization component to pass through it at 100% while permitting no penetration of S-polarization component therethrough; during measurements of S-polarization component, on the other hand, it was adjusted permitting penetration of 100% of S-polarization component while blocking 100% of the P-polarization component. The horizontal axis of both graphs represents the direction of view line b as defined by use of the output angle .phi. shown in FIG. 4 (this is a generalized indication for .phi.2 in FIGS. 2 and 3).
From the graphs of FIGS. 5 and 6, the following can be seen.
1. The angle providing a peak of illuminance is different by several degrees depending upon the polarization component contained.
2. The P-polarization component is greater in luminance peak than S-polarization component.
These facts are not limited to the examples using the light-scatterable optical guide but may also be true for those with light guide plates with ordinary emission directivity.
Thus, in order to optimize the prism sheet 4 in an arrangement of the type shown in FIG. 3, the foregoing polarization characteristics should be taken into consideration. However, no technical idea has been proposed until today to attain such optimization of the prism sheet 4 by fully taking account of the polarization characteristics of illumination light.
A discussion will then be made as to the profile of the prism array of the prism sheet, which concerns the problem (2) above. As has been described with reference to FIG. 3, the outgoing light 4f toward the front direction is mainly derived from the slopes 4d. Accordingly, insofar as the inclination angle .phi.6 of slopes 4d is set adequately, generation of output light 4f can be guaranteed.
On the other hand, as will be described later, the optimal value of the inclination angle .phi.6 for achievement of well-controlled frontal output light 4f remains extremely great; for example, it measures approximately 70.degree.. Hence, under the condition that the profile of each prism in the array resembles the isosceles triangle, the resulting inclination angle .phi.7 of slope 4c must be about 70.degree. accordingly. This results in that the angle .phi.8 of a groove as defined between adjacent prisms is as small as about 40.degree.. It is difficult to precisely manufacture such grooves with the aforementioned fine shape while attaining increased yield of production, which in turn leads to an increase in manufacturing cost. If prism sheets of reduced fabrication accuracy are employed then resultant illumination light can no longer offer uniformity in distribution of intensity, rendering difficult the application to back-lighting for liquid crystal display.